The triples with 583 pistons were 925cc engines and ran very well. To answer your question, yes you have to open up the case for the sleeve skirts. Some sleeves for the 890s will fit but some wont. Depends on the o.d. of the sleeve. Some guys say the 925s don't make anymore power for the work involved compared to a ported stock bore. Get the big bore pipes for the elevation and they run very strong.